Chiun-Hsun Chen

Lipid accumulation and CO2 utilization of Nannochloropsis oculata in response to CO2 aeration.

In order to produce microalgal lipids that can be transformed to biodiesel fuel, effects of concentration of CO(2) aeration on the biomass production and lipid accumulation of Nannochloropsis oculata in a semicontinuous culture were investigated in this study. Lipid content of N. oculata cells at different growth phases was also explored. The results showed that the lipid accumulation from logarithmic phase to stationary phase of N. oculata NCTU-3 was significantly increased from 30.8% to 50.4%. In the microalgal cultures aerated with 2%, 5%, 10% and 15% CO(2), the maximal biomass and lipid productivity in the semicontinuous system were 0.480 and 0.142 g L(-1)d(-1) with 2% CO(2) aeration, respectively. Even the N. oculata NCTU-3 cultured in the semicontinuous system aerated with 15% CO(2), the biomass and lipid productivity could reach to 0.372 and 0.084 g L(-1)d(-1), respectively. In the comparison of productive efficiencies, the semicontinuous system was operated with two culture approaches over 12d. The biomass and lipid productivity of N. oculata NCTU-3 were 0.497 and 0.151 g L(-1)d(-1) in one-day replacement (half broth was replaced each day), and were 0.296 and 0.121 g L(-1)d(-1) in three-day replacement (three fifth broth was replaced every 3d), respectively. To optimize the condition for long-term biomass and lipid yield from N. oculata NCTU-3, this microalga was suggested to grow in the semicontinuous system aerated with 2% CO(2) and operated by one-day replacement.

Diffusion Flame Stabilization at the Leading Edge of a Fuel Plate

Abstract A theoretical model of a laminar diffusion flame at the leading edge of a fuel plate in a forced convective flow is presented and solved numerically to study the flame stabilization and blowoff phenomena. The system of governing equations consists of the two-dimensional Navier-Stokes momentum, energy and species equations with a one-step overall chemical reaction and second-order, finite rate Arrhenius kinetics. The computation is performed over a wide range of Damkohler numbers. For large Damkohler numbers, envelope flames are found to exist where the computed fuel evaporation rate, the flame stand-off distance and the velocity profiles show certain similitude. As the Damkohler number is lowered, a transition to open-tip flame takes place where the flame becomes stabilized on the sides of the fuel plate. Further decreasing of the Damkohler number pushes the diffusion flame downstream out of the leading edge region. In this paper, the flame structures of the envelope and the open-tip flames are p...

Downward flame spread over a thick PMMA slab in an opposed flow environment: experiment and modeling

Abstract This work investigates experimentally and theoretically the downward spread of a flame over a thick polymethylmethacrylate (PMMA) slab with an opposed flow of air. Simulation results, using an unsteady combustion model with mixed convection, indicate that the ignition delay time increases with a decreasing opposed-flow temperature or increasing velocity. The ignition delay time is nearly constant at a low opposed flow velocity, i.e., u ∞ ≤ 30 cm/s . Experiments were conducted at three different opposed flow temperatures and velocities, namely, T i = 313, 333 , and 353 K and u ∞ = 40, 70, and 100 cm/s , respectively. Measurements included the flame-spread rate and temperature distributions, using thermocouples and laser-holographic interferometry. The qualitative trends of the flame-spread rate and thermal boundary layer thickness, as obtained experimentally and from numerical predictions, were identical. For a quantitative comparison, the predicted and experimental flame-spread rates correlated well with each other, except at the lowest velocity ( u ∞ = 40 cm/s) . The discrepancies between the measured and predicted thermal boundary layer thicknesses decreased with an increasing flow velocity. The quantitative agreement with a high velocity indicates that the spread of an opposed flame is mainly controlled by the flame front, whereas the discrepancies at low flow rates demonstrate the importance of radiation, the finite length of the fuel, and also three-dimensional effects, which were not considered in the model. The temperature profiles around the flame front measured by interferometric photographs indicate a recirculation flow there, as predicted by the simulation.

Encapsulated droplets with metered and removable oil shells by electrowetting and dielectrophoresis

A water-core and oil-shell encapsulated droplet exhibits several advantages including enhanced fluidic manipulation, reduced biofouling, decreased evaporation, and simplified device packaging. However, obtaining the encapsulated droplet with an adjustable water-to-oil volume ratio and a further removable oil shell is not possible by reported techniques using manual pipetting or droplet splitting. We report a parallel-plate device capable of generation, encapsulation, rinsing, and emersion of water and/or oil droplets to achieve three major aims. The first aim of our experiments was to form encapsulated droplets by merging electrowetting-driven water droplets and dielectrophoresis-actuated oil droplets whose volumes were precisely controlled. 25 nL water droplets and 2.5 nL non-volatile silicone oil droplets with various viscosities (10, 100, and 1000 cSt) were individually created from their reservoirs to form encapsulated droplets holding different water-to-oil volume ratios of 10:1 and 2:1. Secondly, the driving voltages, evaporation rates, and biofouling of the precise encapsulated droplets were measured. Compared with the bare and immersed droplets, we found the encapsulated droplets (oil shells with lower viscosities and larger volumes) were driven at a smaller voltage or for a wider velocity range. In the dynamic evaporation tests, at a temperature of 20 ± 1 °C and relative humidity of 45 ± 3%, 10 cSt 10:1 and 2:1 encapsulated droplets were moved at the velocity of 0.25 mm s(-1) for 22 and 35 min until losing 16.6 and 17.5% water, respectively, while bare droplets followed the driving signal for only 6 min when 11.4% water was lost. Evaporation was further diminished at the rate of 0.04% min(-1) for a carefully positioned stationary encapsulated droplet. Biofouling of 5 μg ml(-1) FITC-BSA solution was found to be eliminated by the encapsulated droplet from the fluorescent images. The third aim of our research was to remove the oil shell by dissolving it in an on-chip rinsing reservoir containing hexane. After emersion from the rinsing reservoir, the bare droplet was restored as hexane rapidly evaporated. Removal of the oil shell would not only increase the evaporation of the core droplet when necessary, but also enhance the signal-to-noise ratio in the following detection steps.

A Numerical Study of Flame Spread and Blowoff over a Thermally-Thin Solid Fuel in an Opposed Air Flow

Abstract Flame spread and blowoff in an opposed air stream over a thermally-thin solid fuel is studied theoretically. The model includes the quasi-steady, two dimensional Navier-Slokes/ momentum, energy and species equations with one-step overall chemical reaction and second-order, finite-rate Arrhenius kinetics in gas phase. In a reference frame attached to the flame front, the flame spread rate v¯ f) becomes an eigenvalue for this problem. The solid phase equations become steady, consisting of an energy balance coupled with the heat flux from the gas phase and a mass balance including Arrhenius pyrolysis kinetics. The parametric study is based on a variable Damkohler number (Da) which is a function of opposed flow velocity (u∞ ). The spread rate v¯ fand the flame size are reduced and the flame becomes weaker as Da is decreased or u∞ is increased. A blowoff limit is reached when Da is lowered to a critical value. Heat conduction in the solid fuel contributes to higher VF and is the dominant process near ...

Gas-phase radiative effects on downward flame spread in low gravity

Abstract A theoretical analysis is developed to study the effect of radiative heat transfer on downward flame spread over a thin fuel in low gravity. The combustion model, which is an extension of that in Duh and Chen (1991). approximates gas—phase radiation using a two—flux method applied in the cross—stream direction. Numerical results show that there is a quenching limit in the low—gravity region which is inaccessible if radiation is neglected. The main controlling factor in low gravity (g < 0.05) is the conduction to radiation parameter (N∞) where the flame spread rate increases with an increase in Nee. Also, the Darnkohler number alone is found to be insufficient to characterize the flame spread behavior. On the other hand, in higher gravity the Damkohler number becomes dominant. Parametric studies are carried out by changing the gravity level and the ambient oxygen concentration subject to gas—phase radiation. Finally, a flammability map is constructed that combines the two effects.

A THEORY FOR DOWNWARD FLAME SPREAD OVER A THERMALLY-THIN FUEL

Abstract A theoretical analysis is developed to study the downward flame spread over the thermally-thin solid fuel vertically in the gravitational field. The combustion model is basically similar to that of Chen (1990). The effect of Damkohler number (Da), as a function of gravity level (g), is under investigation. Beyond the blowoff limit, the flame spread rate is found to be proportional to (g)−1/3. A combination of the isotherm and velocity vector distributions in gas phase and the solid fuel pyrolysis are presented together to illustrate flame structures at normal gravity. Subject to the same Da, the flame is weaker than that in forced convection. Within the elevated gravity domain of experiment, the computed flame spread rate agree very well with the measurements obtained by Altenkirch el at. (1980).

HEAT TRANSFER FOR INCOMPRESSIBLE AND COMPRESSIBLE FLUID FLOWS OVER A HEATED CYLINDER

The flow and thermal fields in forced convection over a heated cylinder for both incompressible and compressible flows are studied nondimensionally and numerically. The governing system includes fully two-dimensional Navier-Stokes momentum, energy, and continuity equations in body-fitted coordinates. The effect of Reynolds number (Re) is investigated. In the incompressible case, Re is a function of free-stream velocity. The predicted results are in good agreement with those obtained by the other numerical methods and experimental measurements. In the compressible case, Re is governed by cylinder surface temperature. The characteristics of fluid flow and heat transfer when surface temperature is increased are found to be simitar to those obtained by decreasing incoming flow velocity in the incompressible case.

Improvement of CO Tolerance of Proton Exchange Membrane Fuel Cell by an Air-Bleeding Technique

This study investigates the improvement of proton exchange membrane fuel cell (PEMFC) carbon monoxide by periodic air dosing. The carbon monoxide in the fuel gas leads to a significant loss in power density due to CO poisoning in the anode. The method involves bleeding air into the anode fuel stream (H 2 -CO), which contains CO in various concentrations (20 ppm, 52.7 ppm, and 100 ppm). In the transient CO poisoning test, air bleeding is performed for four different periodic air dosing and cell voltage is fixed at 0.6 V. The result of a dosing of air for 10 s in intervals of 10 s is similar to that of continuous air bleeding except for 100 ppm CO. The CO tolerance of the fuel cell and cell performance recovery from poisoning can be improved by air bleeding.

A numerical analysis of ignition to steady downward flame spread over a thin solid fuel

A numerical analysis using an unsteady combustion model is presented to study the ignition and subsequent downward flame spread over a thermally thin solid fuel in a gravitational field. The solid-fuel temperature rises gradually in the heat-up stage and the pyrolysis becomes more intense. Ignition, including the induction period and thermal runaway, occurs as soon as a flammable mixture is formed and the gas-phase temperature, heated by the solid fuel, becomes high enough. During the induction period, the reactivity and temperature in the gas phase are mutually supportive. The thermal runaway consists of a burning premixed flame as the flow moves with the flame front. This is followed by a transition from a premixed flame into a diffusion flame. The flame front extends along and toward the upstream virgin fuel as the diffusion flame is formed. Finally, steady flame spread takes place as burnout appears. The ignition delay time is found to be controlled mainly by the time required to form the flammable mixture and is almost independent of the gravity level and the ambient oxygen index. The ignition delay time increases nearly linearly with an increase in solid-fuel thickness within the range of 0.005\,{\rm cm}\le {\bar \tau}\le 0.02\,{\rm cm} and is proportional to ({\bar Q}_{\max})^{-1.11} within 2\,{\rm W/cm}^2\le\bar Q_{\max}\le 8\,{\rm W/cm}^2 . The steady downward flame-spread rate decreases with increases in the gravity level or fuel thickness and with decreases in the ambient oxygen index but is independent of the incident peak heat flux. The blowoff limit is around 6.7\,{\bar g}_{\rm e} and the extinction limit is found to be Y O X = 0.131.